503 misfire

[Good old number 29 158]

I just diagnosed this most irritating problem and hoped that this solution might benefit some others. 

The 503 in my Kitfox is fresh. It’s jetted right. It has new spark plugs. It has new points. The point gap and timing is good. It has good spark. It has good compression. It has good fuel.

I trailered it to the airpark for some taxi time, but when I got it off the trailer, the number two cylinder (the rear cylinder) was not firing right. I messed with it for a couple hours, but the same thing kept happening. The rear cylinder would miss, then eventually go completely dead. The evidence of this was plain, with low egt and cht on the number two cylinder. The number two spark plug was wet with fuel, while the other was dry. I didn’t have spare plugs with me, but it had a great spark at both plugs. I took both plugs out and spun it over to blow the fuel out. I switched the plugs. I eventually pushed it back to the trailer because it kept doing the same thing. I could clear the cylinder, dry the plug, fire it up, but it would immediately start missing on number two, and eventually that cylinder would stop firing completely. 

I replaced the new NGK B8ES spark plugs with brand new NGK BR8EIX iridium plugs. I checked the carbs and floats. The float needles were properly in place and the floats were floating in the fuel in the bowl, when removed. I pulled out the slides and checked the needles. I lifted up the tail, to level the plane, and fuel came out of the air filter. The fuel valve was off. I double checked the jets, floats, and needles. All good. I started to remove the engine to see if the points were arcing, but the spark looked good, so I put it back together. The points are brand new, with only one hour on the points. The manual recommends readjusting them after ten hours, because they will wear where the cam lifts the breaker to open it. Knowing the gap would be lessening as it wore in, I made the initial adjustment toward the upper end of the range, giving them a wide gap. Certain that the carbs were properly set up, I surmised that it may have flooded the engine while bouncing down the road on the trailer. Being tilted back, the fuel would flood the back cylinder. This was not the case. I put a torch to it and made sure there was no gas in the cylinders. I put it back together. Same thing. Fired up, starts missing, number two takes a number two. So, back to the points. I started taking the engine off again. As I worked, I noticed gas on the floor beneath me. The fuel valve was off. The fuel lines, I left connected, and removed the carbs from the engine. They weren’t leaking out of the vent tubes or overflow. I had taken the primer hoses off of the nipples on the intakes, and it was dripping. I had noticed before that the primer seems to push the fuel back and forth a bit, instead of a steady push in one direction, but the primer had been working, and the engine had been running fine, so I hadn’t thought much more about it. The fuel to the primer isn’t shut off with the fuel valve. There is a tee on the bottom of the panel tank. One side of the tee goes to the valve, the other side goes to the primer. I capped off the fittings and plugged the primer hose. I put everything back on the engine and put the cowl back on. The engine fired up and ran great. Thank God for little things. If this had happened in the air, it would have been a forced landing. Even if the fuel valve could have shut off the fuel to the primer, it wouldn’t have kept it from dripping while the fuel was on. It’s pretty hard to start cold, without the primer.  If I rebuild the primer, I’ll put a fuel valve inline, so I can shut off the fuel to just the primer, so it’s never a problem. Because the plane was tilted back, whether on the trailer or on the ground, the fuel was dripping into the back cylinder and flooding it out. Inflight, it could have drowned them both out. On one cylinder, I could keep it running, but it wasn’t pulling hard enough to move under its own power. Putting in a valve to shut off the primer while it’s running would be a good move, even if you’ve never had a problem. Mine was fine for over thirty years, until that one day it wasn’t. It would be the same with a dual ignition. It was great to hear how smoothly it ran, once I figured it out. Now I have another project to work on, but it’s huge load off my mind. 

[1avidflyer 1983]

Do you have the snowmobile type primer or a regular aircraft one?  I used the snowmobile type most times and they worked ok.  I would put a new one on and start again.  You could run a short section of 1/4" line with a valve in it and then the smaller 1/8" T and lines down stream as well.  JImChuk

[Good old number 29 158]

I haven’t looked into it to see which style it was, but it’s not fixable. It looked like it unscrews, but when I twisted it apart, I found that it isn’t threaded, it was pressed together, metal to metal, without a seal. I don’t think I’ll trust it to seal again now. I don’t think I’ll trust one at all. I’m going to look for a better alternative. I wish these carbs had a regular butterfly valve choke, the ones that actually work. 

[Good old number 29 158]

It was fortunate for me being able to diagnose it, that the fuel valve did not shut off fuel to the primer. I had considered changing the fuel system so the main valve would shut off everything, like it should, but never got to it. Had I done that, I might not have realized that the primer was the problem. Just a small, slow drip was enough to flood it out and kill it. 

[Allen Sutphin 705]

I just replaced a primer  on a T-craft because of the same reason. O rings were gone and it was bypassing fuel and flooding the engine. I suspected ethanol was the reason the O rings were eat up since he had been running auto fuel. Good job of figuring it out!  Funny how we worry about the big things binging us down and it can be something so simple as a tiny O ring.  And with the small plunger type primers sold by LEAF, you'd never know when they are ready to go. A shut off valve in the primer line would solve the issue but what we need is another valve to complicate things.

Edited 1 Dec 2019 by Allen Sutphin

[1avidflyer 1983]

Here is the snowmobile primer I was referring to.  JImChuk

[wypaul 448]

I had the same issues with the primer twice, just gave up on them and went back to just using the enricher. I haven't looked back. I think I still have a new one of those sitting in my spare parts Jim.

[tcj 150]

I'm on the other end of the two stroke primer spectrum.  Ran a 670 Ski Doo with primers for 20 years and a Rotax 503 with a snowmobile primer in my kitfox for 10 years.  In cold weather a couple shots on the primer the Ski Doo started with one pull and two seconds on the electric starter button the 503 kitfox started first blade.

Now, all old Ski Doo drivers know that if you don't shut the fuel valve off when trailering, the needle valves in the carbs will jig, letting fuel through and fill the crankcase with a half a cup of fuel.

[Good old number 29 158]

That was my first thought, even though I did have the fuel valve off. More than a couple times, I’ve trailered snowmachines somewhere, only to get there with one cylinder flooded out, but I’ve never had one flood out like that while it was running. The primer is the same one that Jim posted a picture of. 

[Buckchop 479]

Very interesting, i just mounted a primer on the Bluefox just havent routed the fuel lines yet. So good info to ponder.  My Blackfox  doesnt have one and it starts pretty easy just useing the cable enrichner.  Now i may not put one on. 

[109jb 181]

Those snowmobile primers use a rubber check valve and they can and do fail. It is basically a flexible flapper valve. Older ones are susceptible to ethanol and other additives in modern car gas. I had one on an old Ski-doo 2-up that had turned to a ball of goo and would not prime anything and I suspect that before I got it the previous  owner used ethanol containing gas. I would think newer versions may have different rubber compounds formulated to stand up to the ethanol.  

An aircraft primer has spring and ball check valves and are designed with enough spring tension to not be able to siphon through or to gravity feed through. It also has a pin that locks the check valves in the closed position. If they did leak every Cessna or other high wing airplane out there would be flooded every time you tried to start.

Also from your description, the primer supply is not filtered at all except maybe by the coarse finger strainer in the tank. The primer should come off of the outlet side of the gascolator if there is not a port on top for the primer. This would be downstream of the shutoff valve. If there is a top port on the gascolator this is also filtered  fuel, at least filtered by the gascolator screen.

I know that lots of people use the snowmobile type primers pictured, but personally I would not. An aircraft  primer is about $190 brand new and that I'm willing to pay for peace of mind. Fortunately I don't have to because the builder of my airplane already put an aircraft style primer on it.

Edited 1 Dec 2019 by 109jb

[marksires 484]

Mine has the snowmobile primer, and (when it was flying) I replaced the primer at every condition inspection.  They didn't seem to last much longer than that, and anything that ends up with fuel loose in the cockpit is a bad thing.

[akflyer 1427]

The primers are cheap, I replace mine every year when I replace the hoses as well.  The failures I have seen on mine were leaking fuel around the shaft and dripping on my hand.  You get a feel for how they operate when new, and as soon as they start feeling funny during use (hard to plunge etc) swap them out.  Its an item that is in the go bag in the plane along with spare filters.

 

[Good old number 29 158]

It turned out to be a real bear to start without a primer, so I got one coming. I ordered a shut off valve to go with it.